1. Field of the Invention
The invention relates to new polyamide polyacrylate compositions useful as hot melt adhesives. In particular the invention relates to aminoamide polymer compositions including a reactive diluent which serves to reduce the polyamide viscosity for improved mixing and reaction with the polyacrylate to make a curable thermoplastic resin.
2. Description of Related Art
Polyamides are a well known class of thermoplastic polymer. Most polyamides cannot be cured to thermoset polymers. Certain aminoamide polymers are useful hot melt adhesives, but if they are used in a non-crosslinked thermoplastic mode, they are subject to cold flow, remelting, solvent attack and other modes of deterioration. It is known to crosslink aminoamides, in fact they are often used themselves as crosslinking agents for epoxy resins, but such crosslinking generally requires a "two package" system, the aminoamide resin being one package, and a co-reactive resin, typically an epoxy resin, being the other package. Such "two package" systems are commercially used, but have the shortcomings of requiring two containers, measuring and mixing at the point of use, limited working time after the mixing, and they generally require substantial time and/or heating to effect partial cure, to give good green strength. Even further time and/or heating is required for complete cure.
It is known to blend acrylate polymers into polyamides for improved properties such as for hot melt adhesive uses, as is taught in U.S. Pat. Nos. 4,132,690 (E. Ernstman et al.) and 4,374,231 (Doucet); for molding resin uses as in U.S. Pat. No. 4,247,665 (Daniels et al.); and for pressure-sensitive adhesive uses, as in U.S. Pat. No. 4,696,965 (Rasmussen). However, the acrylate polymers of these disclosures are already polymerized and have no free acrylate groups. Thus, such blends are in most cases not chemically linked by covalent bonds and are not capable of being post cured to render them thermoset (infusible). Grafts and copolymers of polyamides and acrylates, suitable for adhesive uses, are taught in U.S. Pat. Nos. 4,247,665 (Daniels et al.) and 4,378,448 (Part et al.) but these also had the acrylate group used up in the grafting or copolymerization and so free acrylate groups were not available for crosslinking the chains.
Curable adhesives have also been made using acrylate groups as the curing agent as in U.S. Pat. No. 4,153,776 and 4,430,479 (Merton, et al.) but these did not utilize aminoamides.
U.S. Pat. No. 4,329,419 (Goff, et al.) discloses a radiation polymerizable composition of a polyamide ester containing photopolymerizable groups and a solvent for the resin. The improvement was found in the use of a radiation sensitive polymerizable polyfunctional acrylate compound and an aromatic biimidazole photopolymerization initiator. This invention is limited to aromatic polyamides with a high level of mid-chain polymerizable groups and requires a solvent. Not being a hot-melt system, this technology is limited by solvent emission problems. In addition the high aromatic content makes a rigid product.
U.S. Pat. No. 3,127,365 (Floyd) discloses an amino polyamide resin in which an excess of a polyamine is reacted with polymer fatty acids to form a polyamide having unreacted amino groups. The amino polyamides are in turn modified by reaction with acrylic monomers. This invention is a two-component system because upon mixing the acrylate and the high amine number polyamide, the curing reaction takes place. In addition a highly crosslinked product is formed due to the high amine value of the polyamide.
U.S. Pat. No. 4,975,498 (Frihart) discloses acrylate-modified aminoamide resins comprising the Michael addition product of an aminoamide thermoplastic polymer with sufficient polyol ester having a multiplicity of acrylate ester groups so that the modified polyamide contains unreacted acrylate groups. This thermoplastic contains a thermally activatable catalyst to cause curing upon heating.
U.S. Pat. No. 4,987,160 (Frihart) discloses acrylate-modified aminoamide resins which comprise the Michael addition product of an aminoamide thermoplastic polymer with sufficient polyol ester having a multiplicity of acrylate ester groups so that the modified polyamide contains unreacted acrylate groups. This thermoplastic can be cured by radiation, such as electron beam or ultraviolet if a photoinitiation catalyst is added.